Syringes prefilled with a liquid drug (generally referred to as “prefilled syringes”) are used as medical syringes. The prefilled syringes are advantageous because of their handling ease without the need for transferring the liquid drug into the syringe. Further, transfer of a wrong liquid drug into the syringe is advantageously prevented. Therefore, the prefilled syringes are increasingly used in recent years.
Unlike conventional syringes into which a liquid drug sucked up from a vial or other container is transferred immediately before use, the prefilled syringes are each required to serve as a container which is kept in contact with the liquid drug for a long period of time.
Such a syringe typically includes a syringe barrel, a plunger reciprocally movable in the syringe barrel, and a gasket attached to a distal end of the plunger.
The gasket to be used for the syringe is generally made of a crosslinked rubber. It is known that the crosslinked rubber contains various crosslinking components, and these crosslinking components and their thermally decomposed products are liable to migrate into the liquid drug when the liquid drug is kept in contact with the gasket. It is also known that these migrating components adversely influence the efficacy and the stability of some liquid drug.
The gasket is required to be smoothly slidable when the syringe is used. In general, the gasket made of the crosslinked rubber is poorer in slidability. To cope with this, it is a general practice to apply silicone oil to an inner surface of the syringe barrel or the surface of the gasket. However, it is also known that the silicone oil adversely influences the efficacy and the stability of some liquid drug.
From this viewpoint, a product of so-called “laminated gasket” including a rubber gasket body having a surface laminated with a highly slidable film is often used for the medical syringe. Since the surface of the rubber gasket body of the laminated gasket is covered with the highly slidable film, it is possible to prevent the components of the crosslinked rubber from migrating into the liquid drug, and to ensure the slidability even without the use of the silicone oil.
Examples of the film to be used for this purpose include ultrahigh molecular weight polyethylene films and fluororesin films which are excellent in slidability. Of these films, the fluororesin films are advantageous because of their excellent slidability and chemical stability. Of the fluororesin films, polytetrafluoroethylene (PTFE) films are particularly preferred because of their more excellent slidability and stability.